Alpha-l-antitrypsin (AAT) is purified by an immunoadsorbent technique in fully active form. The phenotypic variation is to be studied by attempting to raise in monkeys antisera which are specific for individual variants of the protein. At the same time, monospecific goat antibodies to AAT bound to Sepharose will be used to isolate the active site peptide of AAT which can combine with trypsin and other proteases which complex AAT.